Camera twists issue

Basically, I've produced a 3D box, where a camera stands next and looking at the box. The camera's 'point of interest' & 'position' is expressed towards two nulls. and I've locked two rotation values in the camera, where it does twist when going into the box.

The concept is to animate the camera into the box, up over and then inside of it. The null for 'Point of interest' moves up and down to frame the box. The start of the animation has no problems, when it reaches to the top and when it begins to descends into the box, it starts to twist around.

The desire effect is I want to lock the twist when it gets inside of the box. Is there an expression that I can lock the twist issue? Am I missing something?

If you parent the camera to a 3D null, you can use the standard position and rotation controls on the null to move and rotate the camera without having to worry about animating the camera's position and point of interest separately.

Each method has advantages and disadvantages, but the parenting method sounds like it would be appropriate here.